Update tutorial validation.ipynb

[cecileherr]

1. Update validation.ipynb: textual changes, add code to use a custom id, plot with the vegetation map, move information about upscaling below main explanation There are still problems while I try to run the notebook:

   • error while trying to run the validation with a custom mapping table
   • the png image with the explanation about upscaling do not appear

2. Update niche_vlaanderen\tests\data\bwk_tiny\bwk_clip.dbf (added field myid)

[johanvdw]

I fixed the example with a custom vegetation mapping table. The issue was that the shapefile would contain no niche vegetation types after mapping. We now raise an error for that if it is the case (and I added a test).